Use index for where(XXX.defined => true)

Closes #180

lib/no_brainer/criteria/where.rb

@@ -365,7 +365,7 @@ def get_usable_indexes(options={})
     end
 
     def find_strategy_canonical
-      clauses = get_candidate_clauses(:eq, :in, :between, :near, :intersects)
+      clauses = get_candidate_clauses(:eq, :in, :between, :near, :intersects, :defined)
       return nil unless clauses.present?
 
       usable_indexes = Hash[get_usable_indexes.map { |i| [[i.name], i] }]
@@ -384,6 +384,11 @@ def find_strategy_canonical
             [:get_nearest, circle.center.to_rql, circle.options.merge(options)]
           when :eq      then [:get_all, [clause.value]]
           when :in      then [:get_all, clause.value]
+          when :defined then
+            next unless clause.value == true
+            next unless clause.key_modifier == :scalar && index.multi == false
+            [:between, [RethinkDB::RQL.new.minval, RethinkDB::RQL.new.maxval],
+             :left_bound => :open, :right_bound => :open]
           when :between then [:between, [clause.value.min, clause.value.max],
                               :left_bound => :closed, :right_bound => :closed]
         end

spec/integration/criteria/where_spec.rb

@@ -253,6 +253,20 @@
           SimpleDocument.where(:field2.defined => 'FaLse').count.should == 1
         end
       end
+
+      context 'when using an index' do
+        before { SimpleDocument.index :field1 }
+        before { NoBrainer.sync_indexes }
+        after  { NoBrainer.drop! }
+
+        it 'filters documents' do
+          SimpleDocument.where(:field1.defined => false).count.should == 1
+          SimpleDocument.where(:field1.defined => true).count.should == 2
+
+          SimpleDocument.where(:field1.defined => false).where_indexed?.should == false
+          SimpleDocument.where(:field1.defined => true).where_indexed?.should == true
+        end
+      end
     end
 
     context 'when using undefined' do